Arterio-venous malformations are abnormal connections between arteries and veins that may bypass the arterial microcirculation capillaries (capillary arteries). FIG. 1A is a simplified schematic showing a normal arterial and venous network in which an artery 113 receives oxygen-rich blood 109 from a heart 108 and delivers the blood 109 to a healthy capillary bed 112, which in turn delivers oxygen-rich blood 109 to the surrounding tissues. De-oxygenated blood 114 is received by a vein 115 from the bed 112, for return to heart 108.
FIG. 1B, on the other hand, is a simplified schematic of an AVM 119. An artery 113 receives oxygen-rich blood 109 from a heart 108. Blood 109 may pass from the artery 113 to the vein 115 via one or more pre-capillary connections 118, the blood 109 thereby bypassing the normal capillary network 112. In addition, AVM 119 may include a number of enlarged, engorged vessels 117, including new vessels resulting from angiogenesis. AVM 119 also may result in an increase in the size of one or both of artery 113 and vein 115 proximally of AVM 119, and a decrease in the size of artery 113 and vein 115 distally of AVM 119. Further, AVM 119 may include one or more collateral connections 116 between portions of artery 113.
AVMs also may result in less resistance to blood flow, since the pre-capillary arterioles, which provide resistance to blood flow, are bypassed. Patients with AVMs therefore may have a higher blood flow rate and vessels which are larger than normal (such as shown in FIG. 1B, relative to FIG. 1A). The increase in blood flow may result in a higher cardiac output demand, which may result in patients with AVMs having a higher risk of heart failure. The increase in size and number of vascular channels and the increase in total blood flow to an organ, due to an AVM, also can result in an increase in tissue perfusion and an increase in size and growth of tissue. For example, a child with lower limb AVMs may have increased size and growth of the affected limb, with increased bone growth and limb length discrepancy, resulting in the affected limb being longer and the child walking with a limp. Also, thin walled veins resulting from AVMs are exposed to high arterial pressure, thus making them vulnerable to rupture and bleeding.
AVMs may be congenital or acquired, and progressive, stable, or regressive. AVMs may be cosmetically undesirable, and may result in an increased rate of morbidity and/or mortality. Symptoms associated with AVMs may include hemorrhaging, seizures, mass effect (the mass of enlarged blood vessels causing increased intracranial pressure), pain, swelling, hypertrophy, loss of function, ischemia, embolization, and/or heart failure. AVMs may also be symptomatic or asymptomatic throughout a patient's life. An AVM may occur anywhere in the body, but more frequently occurs in the brain and the legs.
Radiographic imaging of AVMs may pose challenges due to the multiplicity of overlapping and layered blood vessels, which may result in difficulty discerning individual blood vessels from each other, and an inability to determine the direction or flow characteristics in the individual blood vessels and blood flow within the AVM. Using some imaging techniques, such as radiographic imaging, angiography, or CT scan, AVMs may appear as a white cloudy mass where the lumens of the individual arteries and/or veins may not be sufficiently delineated.
Thus, a desire exists to obtain improved patient information relating to an AVM, and to provide techniques to assess risk of an AVM and/or plan and assess treatment of an AVM.